The ability to selectively hydrogenate benzene in the presence of other arenes has been of interest in the refinery industry due to strict government limitations on the concentration of benzene in gasoline products. In the United States, benzene content is currently limited to an average of 0.62% by volume, while in Europe the limit is marginally higher, at 1%. In a refinery, the highest percentage of benzene in the overall gasoline pool comes from the reforming of naphtha into aromatics. The most frequently utilized method of reducing benzene in the gasoline pool is to prefractionate the naphtha to eliminate the C6 component in the reformer feed and thus the amount of benzene formed. However, this approach reduces the feed available to yield gasoline.
A standard method of removing benzene from gasoline streams such as reformed naphtha is extractive distillation. This method utilizes a solvent with affinity for aromatics such as benzene, distills other compounds overhead and recovers an aromatic containing solvent, which can then be separated. Another commonly used method of removing benzene is liquid-liquid extraction. This method utilizes a solvent with an affinity for aromatic molecules. The solvent and the aromatic containing stream are passed in counter-current fashion to recover a solvent rich in aromatics which can be separated. These methods are not selective to benzene. That is, these methods also reduce the amounts of aromatic molecules other than benzene that are present in the stream. It would be beneficial to reduce the benzene content without significantly reducing other aromatic molecule content.
Another industrially important technique is the alkylation of benzene with propene to i-propylbenzene (cumene) carried out using zeolites as an acid catalyst. However, this approach requires significant cleanup of impurities in refinery propene, considerably adding to the required capital cost for commercial use of the technique. Further complicating this technique is the fact that that current commercial metal hydrogenation catalysts (e.g., Pd/Al2O3, Pt/Al2O3, Ni/Al2O3) display marginal reactivity differences between hydrogenation of benzene and other substituted arenes, such as toluene, due to preferential adsorption and hydrogenation on the catalytic surface of substituted aromatics. Furthermore, with Pd/Al2O3 at elevated pressures (e.g., PH2≧about 40 atm) and temperatures (about 110 to about 160° C.) such as are commonly used, minimal selectivity for benzene is observed. The relative hydrogenation rate over Pd catalyst at an equimolar feed composition of 50% toluene and 50% benzene is 0.65±0.10, with little observed dependence on the particular catalyst support or reaction temperature. Additionally, with increasing overall aromatic conversion, selectivity for benzene hydrogenation diminishes. Thus, special engineering designs are applied in commercial refineries to reduce the loss of toluene during benzene hydrogenation processing. Usually this takes the form of separating benzene from toluene by fractionation and hydrogenating the stream comprising benzene in a separate reactor, such as taught by U.S. Pat. No. 5,003,118. However, this approach increases cost due to the additional reactor.
Accordingly, it is desirable to provide methods for the selective hydrogenation of benzene at least over toluene from a complex reactant stream. In addition, it is desirable to provide systems and methods for the selective removal of benzene from gasoline. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.